Supported metal oxides are an important class of heterogeneous catalysts for a variety of chemical applications, including for use in biodiesel production, destructive adsorption of chlorocarbons and chemical warfare agents, and in natural gas upgrading processes such as alkane oxidation, olefin metathesis, and oxidative propane dehydrogenation (ODHP; see FIG. 1).
Structurally, supported metal oxides include one or more metal oxide species loaded onto the surface of an inert support material, whereby the metal oxide species is bonded to the support material. Silica (SiO2), given its abundance and low cost, has traditionally been an important inert support material for such catalysts. Examples of commonly used metal oxides include group 3 metal oxides, such as aluminium oxide; group 4 metal oxides, such as titanium oxide; group 5 metal oxides, such as vanadium oxide, niobium oxide, and tantalum oxide; group 6 metal oxides, such as chromium oxide, molybdenum oxide, and tungsten oxide; and group 7 metal oxides, such as rhenium oxide.
At low metal oxide loadings (i.e, at loading levels insufficient to form a complete metal oxide monolayer on the support surface), the metal oxide species bonded to the support material surface are two-dimensional (typically monomeric, but in some case, oligomeric) species that are dispersed throughout the support material surface. As illustrated in FIG. 2, group 5 metal oxide monomers form monoxo structures, group 6 metal oxide monomers form dioxo structures, and group 7 metal oxide monomers form trioxo structures. Each of these structures exhibits tetrahedral geometry around the central metal atom when supported on silica, although the tetrahedron is somewhat distorted in the group 6 and group 7 monomers.
At sub-monolayer loadings, increased metal oxide loading generally results in increased dispersion (i.e., greater surface density) of the metal within the metal oxide catalyst. This increased dispersion generally increases the efficiency of the catalyst, because more catalytic sites are present within a given surface area of the support. However, at higher loading levels, metal oxide species in the form of three-dimensional nanoparticles begin to form on the support surface. As shown in FIG. 3, such nanoparticles exhibit non-tetrahedral geometry around the central metal atoms. The presence of such nanoparticles can be detrimental to the activity of the catalyst, because the presence of nanoparticles increases the rate of side reactions that compete with the desired reaction, thus reducing the yield and efficiency of production of the desired product.
For example, referring now to FIG. 1, which illustrates the production of propylene by oxidative propane dehydrogenation (ODHP), k1 shows the desired production of propylene from propane, while k2 and k3 show the combustion of reactant and product, respectively, to form COx, a competing side reaction that decreases the efficiency of production of the desired product. The presence of metal oxide nanoparticles on the support surface detrimentally increases the rate of these combustion side reactions. Thus, ideally, metal oxide catalysts are deposited onto the support at a level low enough such that the support surface remains substantially free of metal oxide nanoparticles, yet still at a level high enough to maximize surface density of the monomeric metal oxide species.
Metal oxides on silica, however, have only been able to form dispersed species without nanoparticle formation at very low metal loadings, as compared to other commonly used support materials. Specifically, the highest reported value for dispersion of vanadium on silica is only about 3 vanadium-atoms/nm2, while on other support materials (e.g., Al2O3, TiO2, and ZrO2), dispersed vanadium can exist at surface densities as high as 9 vanadium-atoms/nm2. Similarly, the highest reported value for dispersion of niobium on silica is only 1.1 niobium-atoms/nm2, and the highest reported value for dispersion of tantalum on silica is only 0.8 tantalum-atoms/nm2. These relatively low maximum dispersion densities using silica as a catalytic support have been attributed to the low reactivity of the surface hydroxyls of silica.
Accordingly, there is a need in the art for methods of increasing the low maximum dispersion thresholds for metal oxide catalysts supported on silica surfaces, and for substantially nanoparticle-free catalysts comprising highly dispersed metal oxides on silica supports.